1. Field of the Invention
The field of the present invention is related to resonators, particularly resonators that can be combined into filter structures. More particularly, the present invention relates to evanescent resonators.
2. Description of the Related Art
Resonators are known in the art as devices comprising conductive enclosures, cavities, or wave transmission line sections of a two terminal type. The inductance and capacitance is typically distributed, and the line sections being terminated in other than the characteristic impedance of the line sections, so that the device exhibits resonant characteristics to the existing source of wave energy. Resonators can be used to form band pass/band stop filters to permit/block transmission of a particular range of frequency signals, and filter out unwanted frequencies or noise that can be present in the microwave signals. The resonator cavity is normally designed to have a predetermined cross sectional shape so as to permit resonance at a particular desired frequency. Evanescent resonators are typically constructed from lengths of below-cutoff (e.g. dispersive) transmission line with the resonators formed by posts, capacitive screws, ridges, etc. U.S. Pat. Nos. 6,137,383 and 6,154,106 (which are hereby incorporated by reference as background material) to De Lillo disclose multilayer evanescent resonator devices using via hole technology, wherein the resonator is constructed of dielectric material with resonator holes, that may or may not be filled with air or another gas. There are a plurality of resonators arranged in a single device, typically in an array, that are internally connected.
An evanescent resonator according to an aspect of the present invention includes a single length of evanescent transmission line, terminated in short circuit, and filled with air or a low dielectric constant, and supported by air or a low dielectric constant material. The evanescent resonator is fed by surface wave lines operating at relatively low frequency, which have been dielectrically loaded with a material having a dielectric constant higher than the low dielectric material either filling the evanescent line or supporting the evanescent line. The dielectric constant of the low dielectric material can range approximately from values of 2 to 10. The high dielectric may have a dielectric constant ranging approximately from 4 to 400, although typically 10 to 90 may be preferable, depending on the specific need. Thus, the ratio of the high dielectric constant to low dielectric constant may range, for example from 2 to 200, depending upon the specific dielectric constant of the materials selected. There may be many different values, high or lower, which are particularly dependent upon the dielectric constant of the materials.
According to another aspect of the present invention, the dielectric loading of the surface wave or other feed line permits simulation of the effect of higher frequencies present at the input to evanescent resonators, by decreasing the wavelength to that of the simulated high frequency. Thus, a small evanescent resonator is able to support excitation by the relatively low frequency rather than the high frequency, without requiring compensation by the use of a large resonating capacitor. The incoming wave needs to be foreshortened relative to the wavelength in the medium filling the evanescent section.
According to another aspect of the invention, the evanescent resonator is an individual resonator connected externally to a feed network and wave guiding structure. The feed network is reduced in size by dielectric loading so that the wavelength of the feed network is not much larger than the cutoff wavelength of the resonator structure. One advantage of this aspect of the present invention is that the evanescent resonator is operable at frequencies near (but below) cutoff, but without the reduction in unloaded Q intrinsic to waveguide structures known heretofore.
According to an aspect of the invention, dielectric-loaded feed lines (for example, surface wave lines similar to Goubau lines) and below cutoff air-filled cavities can be used to form L-C sections. The capacitance in the L-C sections is primarily from electric field coupling of the feed line dielectric into the below-cutoff section. The inductance results from a combination of inductors in the inductive tee-equivalent circuit for such below-cutoff sections.
According to an aspect of the present invention, dielectric loading is used to shorten the guide wavelength at the input to the evanescent section, so as to increase the effective input inductance. The dielectrically-loaded feed lines may comprise microstrip, CPW, CPS and surface wave structures (Goubau lines), waveguides, etc. The resulting resonant elements according to the present invention are operable at frequencies below 1 GHz with small dimensions.
According to still another aspect of the present invention, the effective unloaded Q for resonators is approximately 400, a significant improvement over evanescent resonators known in the art, for resonators of such small size and low frequency operation.
The evanescent resonators can be connected together into any sort of filter arrangement. Each of the individual resonators contain a section having a closed conductive wall, and this section, while shown in the drawings to be cylindrical, may be any shape (elliptical, rectangular, free form, etc.). One difference in the various possible shapes is the response may be more simple to calculate in some shapes than others.